In recent years, there has widely been used a rear projection-type image display device (hereinafter simply referred to as “image display device”) for projecting image light, onto a rear projection-type screen (hereinafter simply referred to as “projection screen”) at an enlarged scale achieved by projection means such as a projection lens or the like. The image light is modulated by an image signal emitted from a relatively small image light source such as a cathode-ray tube (CRT) display unit, a liquid-crystal display (LCD) unit, a digital mirror device (DMD), or the like.
In the above image display device, generally, the image light projected onto the rear surface of the projection screen passes through the projection screen and is focused in the vicinity of the front surface thereof, and the focused image light is recognized by the observer in the front direction of the image display device.
FIG. 13 shows an example “a” of the projection screen of a conventional rear projection-type image display device. The projection screen “a” has two sheet-like members closely spaced from each other by a suitable distance.
When the observer views the image display device, an angular distribution of horizontal viewing positions of the observer is greater than an angular distribution of vertical viewing positions of the observer. Therefore, the rear projection-type image display device is required to obtain emitted light in a wider angle in the horizontal direction than in the vertical direction.
The two sheet-like members of the projection screen “a” include, respectively, a lenticular lens sheet “c” having a lenticular lens “b” including a number of cylindrical lenses b1, b1, . . . extending vertically on its rear surface, and a Fresnel lens sheet “e” closely spaced a suitable distance from the lenticular lens sheet “c” and having a Fresnel lens “d” on its surface (front surface) facing the lenticular lens sheet “c”.
Image light is emitted from an image light source (not shown) and projected at an enlarged scale by projection means onto the rear surface of the projection screen “a”. The image light is converted into parallel rays of light by the Fresnel lens “d” of the Fresnel lens sheet “e”, and then the parallel rays of light are focused by the lenticular lens “b” of the lenticular lens sheet “c” onto a number of linear focused points f, f, . . . extending vertically at positions near the front surface of the lenticular lens sheet “c” (see FIG. 13). The image light focused onto the focused points f, f, . . . is spread in the horizontal direction wherein the distribution of viewing positions of the observer is greater. The Fresnel lens “d” of the Fresnel lens sheet “e” is also effective to improve a reduction in luminance at the four corners of the projection screen “a”.
The lenticular lens sheet “c” and the Fresnel lens sheet “e” are of a structure in which the lenticular lens “b” or the Fresnel lens “d” formed of ultraviolet (UV)-curable resin is laminated on a base of acrylic resin, or a structure in which the lenticular lens “b” or the Fresnel lens “d” and the base are integrally formed of acrylic resin.
With the conventional rear projection-type screen such as the projection screen “a” described above, it is ideal that the sheet-like lenses be held in contact with each in the central region thereof.
Generally, however, the sheet-like members such as the lenticular lens sheet and the Fresnel lens sheet are coupled integrally together by tapes applied to peripheral edges thereof. Consequently, the sheet-like members tend to be spaced from each other in the central region that is farthest from the peripheral edges. If the sheet-like members of the rear projection-type screen are too spaced from each other, then the image light may be focused into double images, which make the image blur.
With the conventional rear projection-type screen such as the projection screen “a” described above, as shown in FIGS. 14 and 15, it is customary to form one of the sheet-like members, e.g., the lenticular lens sheet “c”, as a warped sheet-like member, place the warped lenticular lens sheet “c” over the other sheet-like member, i.e., the Fresnel lens sheet “e”, apply a pressure to superpose the peripheral edge of the lenticular lens sheet “c” on the peripheral edge of the Fresnel lens sheet “e”, and apply tapes “g” or the like to their peripheral edges to secure the lenticular lens sheet “c” and the Fresnel lens sheet “e” integrally to each other (see FIG. 15).
In the projection screen “a” shown in FIG. 15, since the lenticular lens sheet “c” is originally of the warped shape, after the lenticular lens sheet “c” and the Fresnel lens sheet “e” are integrally combined with each other, the lenticular lens “b” and the Fresnel lens “d” remain in contact with each other in the central region that is remotest from the secured peripheral edges thereof. Therefore, the lenticular lens sheet “c” and the Fresnel lens sheet “e” are prevented from being too spaced from each other.
With the sheet-like member such as the lenticular sheet lens c being warped beforehand, however, the lenticular lens sheet “c” and the Fresnel lens sheet “e” are possibly pressed against each other under strong forces in the central region. When the lenticular lens sheet “c” and the Fresnel lens sheet “e” are pressed against each other under strong forces, the lens elements of the lenticular lens “b” and the Fresnel lens “d” are strongly pressed against each other, tend to be deformed in shape, and changed in optical characteristics. As a result, the path of the emitted light may be changed to focus the light into a blurred image. In particular, Fresnel lenses are easily deformable when pressed because they are often made of a relatively soft UV-curable resin laminated on a base of acrylic resin.
Even if the lens elements of the lenticular lens “b” and the Fresnel lens “d” are not held in contact with each other under forces strong enough to deform them, when they are continuously vibrated as during shipment, the lens surfaces may be rubbed and scratched due to the contact between the lenses. The scratch may be liable to cause the projection screen “a” to display double or triple images thereon.
According to one solution, as shown in FIG. 15, the conventional rear projection-type screen such as the projection screen “a” has spacers h, h, . . . . The spacers h, h, . . . are suitable shape and size sandwiched between the lenticular lens sheet “c” and the Fresnel lens sheet “e” along the peripheral edge of the projection screen “a” through which the image light does not pass, i.e., which is outside of the effective screen area. The spacers h, h, . . . keep the lenticular lens sheet “c” and the Fresnel lens sheet “e” spaced from each other over their entire area, thereby keeping the lenticular lens “b” and the Fresnel lens “d” out of contact with each other in the central region.
However, since the spacers h, h, . . . are positioned in the peripheral edge that is located outside of the effective screen area mostly remotely from the central area of the projection screen “a”, the method using spacers h, h, . . . poses a limitation on the ability to keep the lenticular lens “b” and the Fresnel lens “d” from contacting each other.
In view of the above problems, it is an object of the present invention to provide a rear projection-type screen including two sheet-like members for use in a rear projection-type image display device. The two sheet-like members are spaced an optimum distance from each other within an effective screen area to prevent lenses of the two sheet-like members from being deformed and scratched for increasing the quality of images displayed thereby.